1. Field of the Invention
The present invention generally relates to a constant voltage circuit of a whole category of electronic equipment aboard a computerized personal organizer, a handset, a voice recognition device, a voice memory device, and a computer, etc.
2. Discussion of the Background
At present, energy saving has been actively promoted to protect environment. For battery-powered electrical equipment, such as mobile phones, digital cameras, etc., energy saving is especially important to prolong battery life. Such portable equipment typically uses a constant voltage circuit.
In constant voltage circuits, in order to quickly respond to fluctuations in output voltage, a bias electrical current, which is hereinafter simply referred to as bias current, in a differential amplifier circuit is typically increased.
However, increasing the bias current causes the constant voltage circuit to consume a greater amount of electrical current.
In view of the foregoing, in a known method, the bias current in the differential amplifier circuit is increased in proportion to the output voltage.
However, phase compensation is difficult in this method because the bias current of the differential amplifier continuously changes. Further, response speed is relatively slow when the output electrical current changes abruptly.
FIG. 1 is a circuit diagram illustrating a known constant voltage circuit that increases bias current of a differential amplifier circuit unit when an output electrical current reaches a predetermined or given value.
As shown in FIG. 1, a constant voltage circuit 100 includes an output transistor M101, PMOS (P-channel Metal Oxide Semiconductor) transistors M102, M103, and M107, NMOS (N-channel Metal Oxide Semiconductor) transistors M104, M105, and M106, a comparator CMP, and resistors R101, R102, and R103.
The constant voltage circuit 100 supplies a drain current of the PMOS transistor M107 that is in proportion to a drain current of the output transistor M101 to the resistor R103. The comparator CMP outputs a high level signal when a decrease in a voltage of the resistor R103 exceeds a reference voltage Vs.
When the comparator CMP outputs the high level signal, the NMOS transistor M106 turns on, which adds a constant current ib from a current source to a bias current ia of the differential amplifier circuit, thus increasing the bias current.
Although the constant voltage circuit 100 shown in FIG. 1 can respond more quickly to abrupt changes in the output electrical current, it is difficult to maintain a high degree of accuracy of the resistor R103 when the constant voltage circuit 100 is an integrated circuit (IC). Consequently, it is difficult to set the output electrical current accurately when the bias current is switched.
The output electrical current when the bias current is switched can be set accurately using a resistor capable of trimming as the resistor R103. However, using such a resistor will increase the cost because an IC chip having a broader area and capable of supporting a trimming process are necessary.
Another known constant voltage circuit detects an output electrical current value based on differences in voltage between both input terminals of a differential amplifier circuit and increases the bias current of the differential amplifier when the voltage difference exceeds a predetermined voltage.
In this constant voltage circuit, because changes in temperature and variations in production process conditions can cause the voltage between a gate and a source of a MOS (Metal Oxide Semiconductor) transistor to fluctuate, it is difficult to set a relation between the output electrical current and the voltage difference between the two input terminals of the differential amplifier circuit accurately.
Further, this constant voltage circuit includes two more differential amplifier circuits in order to measure the voltage difference between the two input terminals of the first differential amplifier circuit, and detects the predetermined voltage using input offset voltage of those two differential amplifier circuits.
However, it is difficult to accurately set the output electrical current at which the bias current is switched similarly to the constant voltage circuit 100 shown in FIG. 1, because the input offset voltage described above fluctuates depending on temperature and production process conditions.